The aim of this project is to elucidate the mechanism of regulation of cholesterol biosynthetic enzymes and the levels of membrane cholesterol in cultured mammalian cells. This is to be done through the biochemical and genetic characterization of somatic mammalian cell mutants resistant to killing by 25-hydroxycholesterol in cholesterol poor medium. Genetic characterization will proceed through the analysis of complementation and dominance characteristics of these mutants in fused cells. Biochemical analysis will include characterization of cytosolic and nuclear receptors for 25-hydroxycholesterol, studies on the rate of the HMG-CoA reductase turnover and studies on the regulation of HMG-CoA reductase messenger RNA synthesis.